Trysail

Trysail. A reduced sail used by small craft in lieu of their main-sail during a storm. Also, a fore-and-aft sail, set with a boom and gaff, in ships, synonymous with the spencers of brigs and schooners, and the spanker or driver of ships. A Storm Trysail  was a fore-and-aft sail, hoisted by a gaff, but having no boom at its foot, and only used in foul weather.

From The Sailor’s Word-Book